Richie Carmichael (of ESRI's Applications Prototype Lab) posted a Time Navigator custom task for Explorer a while back. The task has gone through several iterations and improvements, and the current version (including source code) can be found at ArcScripts.

There's also a YouTube video posted on the task, and Richie also includes a discussion of it at his blog.

We used it the other week for animating some of the fire spread data that we've been compiling. Here it is in use:

You'll also find a number of other tasks for Explorer at ArcScripts, and we'll be adding a direct link to ArcScripts from the Explorer Resource Center soon.